Nombre or nombre de isn't usually abbreviated in plain sentences in French. Among neat-looking¹ abbreviations, except for “n. d.” which would likely be ambiguous, “nb. d.” appears to be the only possibility (but classified as a retranchement multiple and advised against).

In telegraphic style, outside of a sentence, Nb is however often used for nombre de (cf. @rds example).

Notice that numéro (which is also translated as number and abbreviated # in English) has a standard and sometimes mandatory abbreviation when followed by a number: no (the letter o is found in exponent¹), plural would be nos.

—1. More details in Orthotypographie, or probably any other specialized book on the topic.